In the SSC, as indicated in FIG. 3A, a pseudo-noise (hereinbelow abbreviated to PN) code, which is one of binary codes, is modulated with data, and a carrier modulated with the modulated PN code is transmitted. In the figure, reference numeral 31 indicates the data source; 32 is a modulator; 33 is a PN code generator; 34 is a carrier generator; 35 is a modulator; and 36 is an antenna. On the receiver side, as indicated in FIG. 3B, the signal is received and the correlation thereof with a PN code serving as a reference is formed in a matching filter. When the two codes are in accordance with each other and when they appear nearly at same places, the data are restored by processing a self correlation waveform (hereinbelow called correlation spike waveform) having relatively great amplitudes. In the figure, reference numeral 38 is a correlator; 39 is a reference PN code generator; 40 is a data demodulator; and 41 represents the data.
The convolver can be cited as one of the possible matching filters. A convolver is a functional element effecting convolute integration and if the reference binary code (hereinbelow called simply reference code) is in a relation of being inverted in time with respect to the received code, it serves as a matching filter effecting the correlation operation.
As an example of the convolver, there is known a surface acoustic wave (hereinbelow abbreviated to SAW) convolver. SAW convolvers can be classified from the viewpoint of the construction into: (1) those in which an air gap is disposed between a piezoelectric layer and a silicon layer; (2) those in which a piezoelectric layer is formed in one body with a silicon layer through an oxide layer; (3) those composed uniquely of a piezoelectric layer; etc. All of them effect multiplying operations based on the interaction of the two signals, while utilizing non-linear characteristics thereof and the results thus obtained are integrated at an electrode called a gate disposed on an interaction region of the convolver.
FIG. 4 shows an example ilustrating the construction of an SAW convolver. In the figure, reference numerals 42 and 43 are transducers; 44 is a piezoelectric layer; 45 is an oxide layer; 46 is a silicon layer; and 47 is a gate electrode. A signal s(t) inputted through the transducer 42 propagates towards the right and the other signal r(t) inputted through the transducer 43 propagates towards the left. The non-linear characteristics which the piezoelectric body--oxide layer--silicon structure has give rise to the interaction between the signals s(t) and r(t), by which the multiplying operation is effected, and the result thus obtained is integrated by the gate electrode 47.
The signal c(t) outputted by the gate electrode 47 is expressed by the following equation; ##EQU1## where A represents a constant; T the time necessary for making acoustic wave pass through under the gate electrode (hereinbelow called in-gate delay time); x the distance measured in the propagation direction of the signal s(t); and v the sound velocity.
Prior art SSC methods are disclosed in Japanese Patent Documents JP-A-61-280135 and JP-A-63-18835. For example, it is conceivable to use not only various kinds of m code (maximum length linearly occurring code) sequences but also the initial phases of the m code sequences on both the transmitting and the receiving sides as the communication channel dividing means, in the case where the m code sequence is used as the spreading code, in the direct spreading SSC using a correlation such as an SAW convolver. The algorithm, etc. for determining the data demodulation timing and the initial phase information of the m code sequences is described in Japanese Patent Documents JP-A-63-95744.
FIG. 2 is a block diagram illustrating the construction of the code generating device indicated therein.
In FIG. 2, SR.sub.1 .about.SR.sub.n are flipflops constituting a shift register; E.sub.1 .about.E.sub.n are exclusive logic sum gates; G.sub.1 .about.G.sub.n are steering gates for giving the flipflops stated above the intial values therefor; MPX is a multiplexer of a 3-state output; L.sub.1 .about.L.sub.5 are latch circuits; AND.sub.0 .about.AND.sub.n are AND circuits; DE-MPX is a demultiplexer; INV.sub.1 and INV.sub.2 are inverters. Further FBCNT is a control signal for controlling the 3-state output in an enable or a disable state and L.sub.6 is a latch circuit for effecting the state control of the 3-state output in synchronism with the STB signal. Still further STB is an initializing signal for the m series code generation; CS is a chip select signal; LE is a latch enable signal; DAT.sub.1.about.n are data; SEL.sub.0 and SEL.sub.1 are data select signals; FB0 is a feedback input terminal; FB1 is an input terminal for the first stage steering gate; and PN is a code output.
FIG. 5 shows a connection scheme, in the case where the code generator indicated in FIG. 2 is used. TABLE 1 shows the correspondence between the terminals a, b, c and d in FIG. 5 and the name of the signals in FIG. 2.
TABLE 1 ______________________________________ PNG CODE GENERATOR ______________________________________ a FB0 b FB1 c FB2 d PN e CAS ______________________________________
The code output PN is used as the code output (A) of the PN code generator 33 in FIG. 3A on the transmitting side and inputted in the SAW convolver on the receiving side as the received PN code included in the received signal s(t). Further it is used as the code output (B) of the reference PN code generator 39 in FIG. 3B on the receiving side and inputted in the SAW convolver on the receiving side as the reference PN code included in the reference signal r(t).
In the case where a PN code generator (hereinbelow abbreviated to PNG) by the method described above is used for the SSC stated above, since the code output is obtained from the first stage of a plurality of stages of the shift register, i.e. since it is necessary to set different values for the initial phase information of the transmitted PN code, and the initial state of the shift register in the receiving PNG, i.e. the initial phase information of the received PN code, it has the problem that the transmission and reception switching-over speed is reduced, depending on the processing speed of an external circuit (e.g. microprocessor).
This problem exists not only in the case where the code output is obtained from the first stage of the shift register but also in the case where it is obtained from the other stages except for the last stage of the shift register.